1. Field of the Invention
The present invention relates to optical elements used for optical communication, optical wiring, or optical storage, optical integrated devices each formed by integrating the optical elements, optical information transmission systems, and manufacturing methods thereof.
2. Description of the Related Art
Optical communication and optical storage devices have been widely used in recent years. Therefore optical elements have been increasingly in demand. In particular, following the practical use of wavelength multiplexing of optical communication, the use of optical elements for metro and access networks have been increased instead of so-called backbone network systems which are primarily formed using optical fibers. Therefore, as optical elements used in add-drop multiplexers for optical signals, instead of elements formed in combination of bulk materials, use has been increasingly made of planar waveguides which are reduced in size and made of thin optical material films and can be easily integrated with various elements.
The planar waveguide is generally formed of a quartz or silicon substrate and an amorphous material such as silicon oxide provided thereon. As a method for forming a thin film, for example, use is made of a flame deposition method in which a thin film is formed by depositing a material using flame treatment or a vapor-phase growth method such as CVD. The thin film obtained is formed into a predetermined shape by a reactive ion etching method or the like, so that an optical element is obtained.
In order to effectively control light including modulation and switching of optical signal, it is necessary to use a physical effect which is obtained through the interaction between an exterior input signal such as electricity or heat and a material forming an optical element. As a silicon-based planar waveguide, for example, an optical switch using a thermooptic effect and comprising a directional coupler provided with a heater, and a switch formed in combination with MEMS have been known. Unfortunately, both types of switches have slow response, in the microsecond range. An optical switch using a thermooptic effect consumes a large amount of electric power, and an MEMS type switch has a complicated structure and is expensive.
Because of its high speed of response, low power consumption due to voltage drive system, and a simple structure, an electrooptic effect in which the refractive index is changed by the interaction between an electric field and a material has been used for an optical modulator. In order to form an optical modulator using LiNbO3, a Mach-Zehnder waveguide is formed on a single crystal LiNbO3 substrate by the use of a Ti diffusion method and is combined with electrodes. When a voltage is applied to the electrodes, the refractive index of the waveguide can be changed, and as a result, ON/OFF operation of an optical signal can be performed. However, since it is required to use a single crystal substrate, the modulator described above is expensive. Further, since the length of the waveguide must be increased due to a small electrooptic effect of LiNbO3, the size of the element is extremely and problematically increased to several centimeters.
Pb1−xLax(ZryTil-y)1−x/4O3 (PLZT) as a transparent ceramic has a larger electrooptic coefficient by approximately two orders of magnitude than that of a LiNbO3 single crystal and has recently been used to form an optical modulator. Therefore, the size of an optical element may be decreased, and consequently, reduction in cost, lower power consumption, and higher speed of response can be expected. Accordingly, investigation of sol-gel processes for forming thinner films of PLZT have been carried out (see K. D. Preston and G. H. Haertling, Appl. Phys. Lett. 60 (1992), p. 2831, and K. Nashimoto, K. Haga, M. Watanabe, S. Nakamura and E. Osakabe, Appl. Phys. Left. 75 (1999), p. 1054).
However, in order to form a thin film having a high transmittance and a high electrooptic effect, epitaxial growth must be performed, and a single crystal substrate must also be used as an underlying material. Accordingly, waveguides other than silicon-based waveguides are not easily formed on a substrate. In addition, a film-forming process in accordance with a sol-gel method must be performed for a long time in order to obtain a film having the thickness required for an optical element. Therefore, the cost is disadvantageously increased.
Electrooptic materials such as LiNbO3 and PLZT are ferroelectric materials. This property appears when the particular crystal structures of the individual materials are formed. Therefore, when an electrooptic material is used for an optical element, it has been believed that a single crystal substrate thereof must be used or that an electrooptic material must be epitaxially grown on a single crystal substrate.
Hereinafter, the formation of nanophotonic devices which can realize the integration between light and electronics on one chip will be further desired as a significant innovative technique. In order to realize the devices described above, a technique in which an LSI such as a central processing unit (CPU) or a memory and an active optical element such as an optical switch are formed on the same chip must be developed. In addition, a technique in which an electrooptic material such as PLZT is deposited with high crystallinity on a silicon or a quartz substrate must also be developed.
On the other hand, as a novel ceramic film-forming method an aerosol deposition (AD method) has been developed. The AD method is a film-forming method based on an impact consolidation phenomenon and utilizes the phenomenon of collision and coalescence of ultra fine particles. Compared to a related thin film-forming method, the AD method is expected to realize higher film-forming rates and low process temperatures (see Jun Akedo and Maxim Lebedev, “MATERIA”41, (2002), P. 459 and Jun Akedo and Maxim Lebedev, Jpn. J. Appl. Phys. 38, (1999), P. 5397). Moreover, in the AD method, the properties of a film are independent of any underlayer, and therefore, a material for the substrate can be optionally selected.
A technique disclosed in Japanese Unexamined Patent Application Publication No. 2001-3180 is a film-forming technique in accordance with the AD method and is characterized in that ultra fine fragile particles provided on a substrate are pulverized by applying mechanical impact so that the ultra fine fragile particles are bonded to each other or that the ultra fine fragile particles are bonded to each other and are bonded to the substrate. As a result, the bonding between the ultra fine particles is realized, and without applying heat, a film having a high density and a high strength can be formed.
A technique disclosed in Japanese Unexamined Patent Application Publication No. 2002-235181 relates to the structure formed in accordance with the AD method and is characterized in that the structure is a polycrystal material in which no crystal orientations exist and in which grain boundaries made of glass layers are not substantially present. In addition, this structure is characterized in that the average crystal particle diameter is 50 nm or less and in that the compactness is 99% or more. The average crystal particle diameter disclosed in Japanese Unexamined Patent Application Publication No. 2002-235181 is the size of the crystal particle calculated in accordance with the Scherrer method of X-ray diffraction analysis.
A technique disclosed in Japanese Unexamined Patent Application Publication No. 2002-20878 is a film-forming technique in accordance with the AD method in which ultra fine particles are sprayed onto a substrate surface and is characterized in that at least part of this spray flow is obliquely incident on the substrate surface. As a consequence, a film can be formed in which the ultra fine particles therein are sufficiently bonded to each other, the texture is dense, the surface is smooth, and the density is uniform.
A technique disclosed in Japanese Unexamined Patent Application Publication No. 2001-38274 is a technique for forming a planarized film composed of ultra fine particles in accordance with the AD method and is characterized in that the film formation is carried out by performing at least one planarizing step for planarizing the surface of a film formed by deposition.
As described above, the development of techniques for piezoelectric materials and the like in accordance with the AD method has significantly advanced. However, the application of the AD method to optical elements has not been studied and, with respect to PZT and PLZT thin films, the transparency thereof have only been reported as of today. The transparency of the film obtained by the AD method is 30 dB/mm or more in terms of the transmission loss, and therefore, it is impossible to apply this film for optical elements.